Thermoplastic injection molding compositions which include a polybutylene terephthalate, polyethylene terephthalate, a polycarbonate and from 5-40% by weight barium sulfate and glass fibers are described in East German Patent No. 230,252A. When these compositions are extruded, the melt viscosity increases and the color of the extrudate changes. In copending application Ser. No. 07/424,749, there are disclosed compositions of polybutylene terephthalate; polyethylene terephthalate and from 30-85% by weight of barium sulfate The compositions of application Ser. No. 07/424,749, which have high levels of barium sulfate, were disclosed as having a ceramic like feel and appearance because of the density, and the smooth glossy surface.
When polybutylene terephthalate or polyethylene terephthalate molding compositions which include a polycarbonate resin are extruded, there may be a chemical reaction which causes changes in melt viscosity, crystallinity or color or may give rise to gaseous by-products.
U.S. Patent No. 4,532,290 discloses the use of monosodium phosphate and/or monopotassium phosphate as a stabilizer to prevent undesirable changes in the melting point of a polycarbonate-polyester resin blend.
The applicants have discovered that compositions which are based on a polybutylene terephthalate or polyethylene terephthalate resin, or a mixture thereof with a polycarbonate resin with high levels of a inorganic filler will have improved impact resistance as compared to similar polyester resin composition while retaining a ceramic-like feel. The styrene rubber impact modifier provides impact strength without loss of the ceramic like qualities. Certain impact modifiers like polyolefins are impractical because they have a waxy feel which is unacceptable in many applications.
The present invention is based on the discovery that thermoplastic compositions of a polybutylene terephthalate and/or polyethylene terephthalate and an aromatic polycarbonate and particular inorganic fillers may be molded into an article having ceramic like properties and good impact strength. If a stabilizer selected from the group consisting of a particular class of acidic phosphate salts, phosphites, phosphate salts of a Group IB or Group IIB metal, polyacid pyrophosphates and salts thereof, and phosphorous oxo acids are added to such compositions melt stability and color are improved. The inorganic fillers are selected from the group consisting of barium sulfate, strontium sulfate, zirconium oxide and zinc sulfate. If desired, a styrene rubber impact modifier may be added to the composition of the invention as well as a fibrous glass reinforcing filler. The compositions of the invention have increased resistance to color changes and melt viscosity changes when subjected to abusive molding conditions, or the repeated molding cycles which are required for the reprocessing of scrap. In addition, interpolymer formation is retarded and crystallinity is retained.
Accordingly, it is an object of the invention to provide a polyester-polycarbonate molding composition which has ceramic like qualities and can be molded into relatively thin sections.
It is also an object of this invention to provide a polyester-polycarbonate composition which has ceramic like qualities and an impact modifier which provides exceptionally high impact strength.
These and other objects will become apparent from the present application.